Harnack is best known as a church historian of the first rank, but a case can also be made for his significance as a constructive theologian. This thesis sets out to examine his theology using an interpretive framework not employed before, namely, the theme of power which runs throughout his work. Use is made of the sociological typology of power developed by Dennis Wrong on the basis of Weber, of Michael Mann's work on the origins of social power, and of Peter Berger's classification of various theological approaches to secularisation and religious pluralism. The investigation broadens the base of Harnack research by making fuller use than has generally the case of publications other than Das Wesen and Dogmengeschichte. and pays attention to Harnack's devotional as well as his scholarly writing. The theory of power which emerges is strongly dualistic, distinguishing interior, ideological power from external, social power. Divine power empowers individuals from within, via their acceptance of the charismatic and competent authority of Jesus as legitimate authority, and the concomitant attribution of benign, nutrient power to the character of God. This acceptance produces markedly beneficial effects upon the individual’s emotional and moral state, which gives rise to constructive activity in the external world. Inherent in this theory is a high estimate of the empowering possibilities of ideas and personalities. Harnack argues that the early church underwent a regrettable process Ctranspotentiation'), involving the growth of external, coercive authority and a diminution in the availability of interior, divine/spiritual power. Harnack's work contains serious tensions related to its untenable dualism and its ambivalence about the possibility of legitimate external authority in the church. But his explanation of divine/spiritual empowering provided a plausible, attractive theodicy and apologetic in its time. With modifications it could do so again, and is of especial value in developing an existential theology of the inner life.